Drilling fluids, referred to as drilling mud, are added to a wellbore to facilitate the drilling process in a variety of ways. Using formate brine (sodium, potassium, and cesium salts of formic acid) as drilling mud is more environmentally friendly and is suitable for high temperature (up to 437° F.) and high pressure (1,100 bar) drilling. However, formate brine presents a challenge for formation evaluation by traditional logging technologies. This is especially true for nuclear tools for which the invasion of formate brine results in significant underestimation of formation porosity. Formate brine has a lower hydrogen index (proton density) compared to water in the same volume. Thus, if the low proportion of proton in formate brine is not accounted for, the porosity estimate will be lower than the actual porosity value. Accordingly, a method and apparatus that corrects the hydrogen index of the total fluid and provides a more accurate total porosity estimate would be appreciated in the drilling industry.